According to a reinforcing bar binding machine of a background art for binding a reinforcing bars by feeding a binding wire to form a binding wire loop around the reinforcing bars and thereafter twisting the binding wire loop to bind the reinforcing bars, the binding wire is wound around the reinforcing bars by two or more turns to bind and therefore, there poses a problem that an amount of consuming the binding wire is large. Further, since an amount of feeding the binding wire is constant, when a diameter of the reinforcing bar is slender, an amount of twisting the binding wire is increased, a length of a twisted portion is prolonged and therefore, when concrete is cast thereto, the binding wire is projected from a surface of concrete and a problem may be posed in a finishing.
In order to resolve the above-described drawback, the applicant has already proposed a reinforcing bar binding machine constituted such that one turn of a binding loop is formed by a binding wire feeding mechanism, a front of a binding wire is held by a clamping mechanism and thereafter, a loop diameter of the binding wire is contracted by reversely rotating a motor for feeding the binding wire and thereafter, the binding wire is twisted to bind. However, there are various boldnesses in the reinforcing bar and a pertinent pull back amount is not constant. Therefore, when the pull back amount is constituted to control in accordance with a diameter of the reinforcing bar, the binding can further uniformly be finished. As pull back amount controlling means, an amount of reversely rotating a motor for feeding the binding wire may be adjusted by adjusting means of a dial, a key switch or the like, however, such an manual adjusting means is difficult to set pertinently, further, the amount needs to be adjusted each time in accordance with the reinforcing bar diameter, and therefore, this is not efficient.